I just measured a 30.5 but I feel like xs would be small or I wont be able to breathe right. But I also want my chest to flatten so would the small still do that for me or would it be kinda loose?

If I understand correctly what you refer to, you're right on the edge there. With the 997 I would err on the small side and with the tritop I'd err on the larger side. My 997 stretched a good bit more than the tritop. Also take into account whether you think you'll be gaining weight from T or working out.